APOP.
In order for this method to work, the password must be stored unencrypted.

cram-md5

AUTH CRAM-MD5.
In order for this method to work, the password must be stored unencrypted.

Syntax:

pop3_capabilitiesextension ...;

Default:

pop3_capabilities TOP USER UIDL;

Context:

mail, server

Sets the
POP3 protocol
extensions list that is passed to the client in response to
the CAPA command.
The authentication methods specified in the pop3_auth and
(SASL extension) and
STLS directives,
are automatically added to this list if the
starttls directive is enabled.

It makes sense to specify the extensions
supported by the POP3 backends
to which the clients are proxied (if these extensions are related to commands
used after the authentication, when nginx transparently proxies the client
connection to the backend).

The current list of standardized extensions is published at
www.iana.org.